It’s an unruly quirk that takes years to master, so how does Deku control One For All?
Deku is able to control One For All by spreading it throughout his body. Turning on all his switches at once is easier to control in the long run than choosing specific parts of the body.
He had serious issues using One For All before this realization. Deku would break his bones when trying to center his quirk into one specific point. Once Gran Torino inspires him to spread his quirk out more evenly, he’s actually able to use it as a shield instead of a risky gamble.
Deku learns how to control One For All
In chapter #48, Deku is microwaving taiyaki for Gran Torino, the retired hero he’s doing an internship with. He is trying to make the use of One For All as easy as breathing, but drawing on a percent of power as low as 5 is already putting a ridiculous strain on his body.
Gran Torino takes a bite of the taiyaki, but it’s cold in the center. He explains to Deku that the meal can’t spin if the plate it’s on is too big. Only one part of the food is heated up.
This lesson about microwaving taiyaki sparked an idea in Deku’s mind. Deku explains that up until now, he was too fixated on the idea of using his power only when or where he needed it. He treated it like an on-off switch, so his reactions were always delayed after multiple uses.
He realizes that from the start, the switches all throughout this body should have been turned on at once. The “heat” which was only reaching a single part of Deku needed to be spread around his entire body evenly.
So that’s the answer. Deku is able to control One For All by evenly distributing it throughout his body. If he pinpoints it in one place, his bone will break. Instead, he spreads the quirk out which gives him much more control.